Following the approach presented in [1]}, we have fitted the line profiles by an exponential law of the type \(\exp (-V/V_0)\) , a procedure which is obviously limited by the SNR of the data. In practice, we considered the
lower-\(J\)  transitions of HCN, H\(^{13}\) CN and HNC. The results are displayed in Fig. REF , where the spectra are displayed in a linear-logarithmic scale.
The spectral slope of the HCN (H\(^{13}\) CN) line profiles considered can be fitted by an exponential law of the type \(\exp (-V/V_0)\) ,
with \(V_0\) = \(3.8\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  (\(V_0\) = \(4.5\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\) ; Fig REF ). The similar values obtained for \(V_0\)  indicate that we are indeed probing the same gas in the different transitions of the same molecular species. A small deviation with respect to the fit is observed at \(V < -10\hbox{\hspace{1.99997pt}km\hspace{1.99997pt}s$^{-1}$}\)  in the HCN \(J\) =1–0 line profile. This is probably due to the overlap between the contributions of the different hyperfine satellites. Another possibility would be the presence of spatial gradients of emission across the telescope beam [2]}.
